(I) Cosmic light: Various electromagnetic waves (light) reach the ground from outer space. For example, radio astronomy in which the nature of outer space is studied by observing X rays from outer space is well known. Of electromagnetic waves that reach the ground from outer space, the short wavelength infrared (SWIR) band can be detected without large-scale apparatuses and hence has been attracting attention and often discussed in recent years. For example, observation results of SWIR spectra from outer space have been described and it has been discussed that the spectra have peaks in the range of 1.4 to 1.9 μm (Non Patent Document 1). In addition, for example, a night-vision camera including InGaAs light-receiving elements (In atom:Ga atom=0.53:0.47) whose lattice matches that of InP has been described (Non Patent Document 2). In this case, to make the lattice constant match that of an InP substrate, the atomic ratio of In/Ga is made to be 0.53/0.47. As a result, the long-wavelength limit (sensitivity limit) of the light-receiving elements is about 1.7 μm. In the descriptions below, light in the SWIR band that reaches the ground from outer space is referred to as cosmic light, SWIR cosmic light, or, simply, SWIR-band light.(II) Compound semiconductor light-receiving element: A prototype of a light-receiving element has been announced in which the light-receiving sensitivity range is attempted to be increased to a longer wavelength by using a light-receiving layer of In0.82Ga0.18As in which the Ga content is decreased and the In content is increased in terms of group III and, as a result, the band gap has been narrowed (lattice constant has been increased) (Non Patent Document 3). In such a case where the In content is increased, the lattice constant of InGaAs is increased and the lattice mismatch between InGaAs and an InP substrate is increased. In the above described light-receiving element, this problem was attempted to be solved by disposing 12 to 20 InAsP graded layers in which an (As/P) ratio is increased stepwise to the light-receiving layer between the InP substrate and the high-In-content InGaAs light-receiving layer. An increase in lattice mismatch results in an increase in the density of lattice defects, which inevitably increases dark current. Even when the graded buffer layers are provided, the dark current is 20 to 35 μA. This high dark current is three orders of magnitude higher than that of photodiodes with InGaAs light-receiving layers for optical communications. In addition, epitaxial growth of a large number of graded layers is not easy and increases the production cost.
In addition, a quaternary group III-V semiconductor has been proposed in which GaInNAs is used by further adding nitrogen (N) as another group V element to InGaAs and a decrease in the band gap has been achieved by the presence of N (Patent Document 1). However, it is very difficult to perform the technique of growing GaInNAs crystals, which contain N. In particular, to achieve a light-receiving sensitivity up to a wavelength of 3 μm and to achieve the lattice match with an InP substrate, the amount of nitrogen needs to be increased to about 10% (atomic % in group V elements). However, when the amount of nitrogen is made to be about 10%, it is very difficult to achieve good crystal quality. In addition, to achieve a high sensitivity of a light-receiving element, the thickness of the GaInNAs layer containing nitrogen at a high concentration needs to be 2 μm or more. However, it is more difficult to grow a N-containing crystal layer having such a thickness and good crystal quality.
The fabrication result of a photodiode with a cutoff wavelength of 2.39 μm has been reported in which a type II quantum well structure of InGaAs/GaAsSb is used and a pn junction is formed with a p-type or n-type epitaxial layer (Non Patent Document 4). This document states that, to make the cutoff wavelength longer, distortion compensation is necessary, and proposes a photodetector having a distortion-compensation quantum well structure of Ga(In)AsSb/GaInAs(Sb) and a cutoff wavelength of 2 to 5 μm.
An image pickup device has an array structure in which a plurality of light-receiving elements are two-dimensionally or one-dimensionally arranged. However, unless the light-receiving elements are isolated from each other with certainty, dark current, crosstalk, or the like is caused and clear images are not provided. It is necessary that photodiodes include a pn junction. In the above-described photodiode, the pn junction is formed by, on a p-type semiconductor layer or an n-type semiconductor layer, epitaxially growing an opposite conduction-type semiconductor layer with each other. In this case, to divide a wide and planar pn junction into pn junctions for individual light-receiving elements, trenches for the division into individual light-receiving elements are provided. Such trenches are referred to as element isolation trenches and are formed by mesa etching after the formation of a planar pn junction. In the formation of element isolation trenches in a near-infrared photodiode including an InP substrate, an etchant having selectivity between InP and InGaAs is used. As a result, wet etching can be stopped at the boundary between the layers (Patent Document 2).
However, when such a wet etching process is used, it is difficult to accurately control the shape of light-receiving elements to be provided by the division. For example, light-receiving elements whose longitudinal sections are tapered and have a shape of trapezoid are formed, light-receiving elements in which the side surfaces of laminated bodies have indentations (irregularities) according to semiconductor layers are formed, or light-receiving elements are formed in which an etchant does not sequentially reach regions between light-receiving elements and the formation of complete trenches is not achieved but is stopped midway. It is very difficult to completely eliminate such imperfection of element isolation trenches. Alternatively, when a dry etching process is used, damage is caused during the etching and hence it is difficult to stably produce photodiodes having low dark current. Thus, the yield is degraded and the production cost is increased.
As for the formation of a structure in which a plurality of light-receiving elements are arranged, that is, the formation of an array of light-receiving elements, in the structures proposed in the above-described documents, the pn junctions except for that in Non Patent Document 3 are formed between a p-type epitaxial layer and an n-type epitaxial layer and a one-dimensional or two-dimensional arrangement of light-receiving elements is formed with element isolation trenches. Accordingly, the above-described problem (high dark current) due to the formation of element isolation trenches is caused.
(III) Night vision device: In recent years, night vision devices employing light in the near-infrared long-wavelength range have been proposed. For example, those proposed are an apparatus that enhances the rear vision of an automobile by radiating infrared rays to subjects including human beings and capturing the reflected light with an infrared camera (Patent Document 3); similarly, a night vision device for automobiles in which near-infrared light-emitting diodes (LEDs) and an image pickup device are combined (Patent Document 4); a vision apparatus employing the combination of two wavelength ranges in the infrared region and the near-infrared region (Patent Document 5); an image pickup device mounted on a vehicle in which light in the 1.5 μm band is received with InGaAs light-receiving elements (Patent Document 6); and the like.    [Non Patent Document 1] Vatsia, Mirshri, L. “Atmospheric Optical Environment”, Research and Development Technical Report ECOM-7023, September (1972)    [Non Patent Document 2] Marshall J. Cohen, “Near-IR imaging cameras operate at room temperature”, LASER FOCUS WORLD June 1993 p. 109 (Sensors Unlimited)    [Non Patent Document 3] T. Murakami, H. Takahashi, M. Nakayama, Y. Miura, K. Takemoto, D. Hara, “InxGa1-xAs/InAsyP1-y detector for near infrared (1-2.6 μm)”, Conference Proceedings of Indium Phosphide and Related Materials 1995, May, Sapporo, pp. 528-531    [Non Patent Document 4] R. Sidhu, “A Long-Wavelength Photodiode on InP Using Lattice-Matched GaInAs-GaAsSb Type-II Quantum Wells, IEEE Photonics Technology Letters, Vol. 17, No. 12 (2005), pp. 2715-2717    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 9-219563    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-144278    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2004-350228    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2002-274258    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 9-37147    [Patent Document 6] Japanese Unexamined Patent Application Publication No. 7-302928